The present invention relates to a method of and apparatus for treating chemical pulp to optimize the consumption of bleaching chemicals and improve the quality of the pulp. Especially the invention relates to a method and an apparatus, by means of which filtrate obtained from a suitable washing stage of brown stock preferably produced by an alkaline cooking process is treated with an oxidizing chemical prior to the oxygen stage following brown stock washing.
In the oxygen stage carried out in medium consistency range, the amount of filtrate per one kg of pulp is 6-9 kg, and thus the properties of the filtrate have an essential effect on reactions which the pulp is subjected to in the oxygen stage, as also in the bleaching later on. So, the properties of the filtrate surrounding the pulp may have a significant effect on the chemical treatments carried out on pulp and also the disadvantageous reactions that the pulp is exposed to.
During the cooking, great amounts of organic material, mainly comprising lignin and carbohydrates originating from hemicellulose are detached off the wood fibers. Each of these organic materials has a chemical composition of its own as a result of the cooking conditions. When passing to the washing and the oxygen stage, these organic materials are carrying chemical compounds and end groups, which react with e.g. oxygen and peroxide. Thus, compounds practically inert in cooking conditions are reactive in new chemical conditions.
In most cases the oxygen stage is connected according the counter-current washing principle so that the object of the so-called brown stock washing located between the cook and the oxygen stage is to replace the liquor passed from the cook with the pulp. This liquor may be referred to e.g. washing loss and/or COD-load and is obtained as filtrate from said last washing stage, with filtrate obtained from the washing in the oxygen stage. The latter filtrate has passed through the oxygen stage with the pulp and due to that has an almost insignificant chemical potential to react with the chemicals in the oxygen stage, so that the chemicals may be used specifically for the desired reactions with the pulp. Nevertheless, some amount of black liquor components is always passed through the washing, which components play a different role than the oxidized filtrate.
In this connection, the oxygen stage refers to an alkaline stage carried out pressurized in the pressure range of 1-17 bar (abg.), and pH-range of 8.5-14, in which stage oxygen is present around the fibers at least part of the reaction time. The oxygen stage may have one, two or even more steps, whereby each reaction step comprises a reaction vessel or reaction retention effected with a tube. In practice, reaction step refers in this connection to adding and mixing some chemical used in the oxygen stage and the following retention at the tube portion. A reaction time short when practiced may thus in mathematical modeling lead to oxygen stages having four or even five steps. Reaction retentions are, depending on the applied method, from 0.1 min to 120 minutes, as the reaction retention is dependent on the desired type of reaction. In this connection, the oxygen stage is identified by a washing stage both prior to and after the oxygen stage and the fact that from the filtrate obtained from the washing after the oxygen stage usually at least part or all the filtrate is introduced to the washing prior to the oxygen stage to be used as washing liquid, so that the oxygen stage is connected countercurrently either completely or at least partially.
Most usually, oxygen and alkali and possibly some inhibitor preventing the deteriorating effect of metals on fibers is dosed into the oxygen stage, or the metals traveling with the fibers are otherwise removed or made non-reactive. The alkali charge is usually 1-60 kg ADMT (air dried metric ton) pulp and the oxygen charge 1-50 kg/ADMT pulp. The alkali that is used is most often sodium hydroxide or oxidized white liquor, but in principle all alkaline compounds containing OH-ion are alkalis which might be used in some conditions in the oxygen stage. The oxygen is dosed in gaseous form, the oxygen content most usually being 75-100% of the specific weight. The temperature in the oxygen stage is 70-120xc2x0 C. and in most cases 80-105xc2x0 C. The temperature may be raised utilizing some suitable steam having a pressure of 0.5-20 bar and hot water either via washing or dilution. The steam may be used for heating either mixed directly into the pulp or indirectly.
As to reaction kinetics, the oxygen stage is carried out so that raising the temperature and increasing the alkali charge lead to acceleration of the delignification reaction. The oxygen charge, in turn, is mainly not effected without increasing the amount of alkali. The suppliers of the oxygen stage have their own opinions about which variable is determinant in different steps and thus each supplier regulates the chemical and temperature profile according to his own desire. Nevertheless, as to reaction kinetics, in all applications the kinetics of temperature, oxygen and alkali follow one and the same basic principle.
According to our studies, the chemical reactions of the oxygen stage as a whole proceed essentially so that part of the oxygen reacts directly with the lignin compounds of the pulp and splits lignin by means of a direct reaction. Oxygen in itself is a selective chemical, which does not split carbohydrates. But in alkaline conditions part of the oxygen converts to peroxide which is very quickly decomposed to hydroxyl radicals by the effect of e.g. black liquor compounds originating from the cook. A hydroxyl radical is chemically very reactive, and the reactions thereof are not districted to reacting with lignin only, but it also causes splitting of carbohydrate chains of the pulp. Practice has shown that the selectivity or non-selectivity of a hydroxyl radical may be described e.g. so that a hydroxyl radical splits one cellulose molecule per five lignin molecules. In our experiments especially the presence of black liquor increased the degradation of peroxide and, accordingly, accelerated the forming of hydroxyl radicals at the end of the reaction chain, whereby a bigger portion of the oxygen changes via peroxide to hydroxyl radicals and thus causes damages to the pulp.
When elaborating the oxygen delignification following the washing of chemical pulp, the operation of the brown stock washing line, located in the process order prior to the oxygen stage, is usually determined so that the washing losses have to be adequately low before the oxygen stage in order to obtain a satisfactory selectivity. The term washing loss is used to refer to impurities remaining in the pulp despite the washing, which impurities in this case comprise both different chemicals and organic materials dissolved in the liquid phase during the cook. Various producers of apparatuses have different opinions on an acceptable level of washing losses. Nevertheless, prior art has not earlier performed any systematic reporting about any chemical mechanism or reason to why different washing loss levels have in different mills resulted in contradictory results concerning the effect of the impurity of the pulp on e.g. viscosity and strength properties of the pulp. This invention is based on extensive comparative studies, in which at least one significant reason for the quality losses of pulp has been determined and thus chemical reasons for quality losses of pulp found. According to said studies, the quality losses of pulp are generated as a result of the following kind of process:
The conditions in the oxygen stage generate peroxide as oxygen decomposes in alkaline condition.
Peroxide decomposes to hydroxyl radicals.
The presence of non-oxidized black liquor originating from the cook catalyses and accelerates the forming of hydroxyl radicals.
The hydroxyl radicals, due to their low selectivity, split cellulose molecules and thus cause quality losses.
In mills especially the washing loss level varies, whereby black liquor entering the oxygen stage in form of washing losses causes fluctuations in the quality.
In our studies we have noted that if the filtrate surrounding the fiber is oxidized e.g. so that it has been separately oxidized prior to feeding it into the pulp in such a way that as much as possible of the liquor around the fibers is oxidized the strong catalytic effect of black liquor originating from the cook is eliminated at the same. When as much of the liquor in the pulp has been oxidized, the quality of the pulp remains higher. Especially after 20-30 minutes the delignification proceeds selectively, even though the advantage of selectivity may be noticed right in the beginning of this stage, so that the oxygen stage may in every case be utilized in more efficient conditions than in cases where the cook-originating catalyte is present.
Prior art knows a plurality of various applications treating the filtrates of the pulp manufacturing process with an oxidizing chemical. In the methods of prior art, presented e.g. in patent publications WO-A-99/29599, EP-A-0 564 443 and FI-A-96156, the filtrate obtained from the washing following the oxygen/bleaching stage is treated with an oxidizing chemical, after which the filtrate is used as washing liquid in the wash preceding the oxygen stage. FIG. 1 illustrates as an example of prior art the solution of FI patent application 961856. The basic principle of the method described in said publication is not to prevent organic loading from entering the stage, but to decrease effluents and ensure the level of oxidizing in the circulating liquor.
Most usually prior art methods have aimed at either removing heavy metals from the filtrate obtained from pulp washing by oxidizing in order to prevent said metals from hampering e.g. the peroxide stage, or at the common to close the bleaching system of the pulp mill. Said FI publication concentrates specifically on treating the filtrate of the peroxide stage. It has been noticed that in some cases the brightness of the pulp is adversely effected when the washer following the peroxide stage discharges yellowish filtrate, which then is returned as washing liquid to the washer preceding the peroxide stage. In other words, the impurities causing the yellowish color, especially organic impurities, are recirculated back upstream of the peroxide stage. In the invention presented in said publication reveals that the yellowish color of filtrate/washing liquid may be eliminated, if the filtrate, or more exactly the organic impurities therein, are oxidized prior to returning it as washing liquid back to the washer preceding the peroxide stage. The publication suggests exhaust gas of the ozone bleaching stage to be used for the oxidizing, which exhaust gas typically contains the oxygen acting as carrier gas in the ozone stage and some residual ozone. The method according to this publication is strongly related to TCF-bleaching and participates in eliminating many problems related to TCF-bleaching.
It is our understanding that in industrial solutions, separate treatment of the filtrates of the oxygen stage with a chemical has not been performed, though. There have often been various correlations on the effect of washing losses determined by COD (chemical oxygen consumption) analysis illustrating organic washing loss on the operation of the oxygen stage as well as the quality parameters of the pulp, but the information has often been contradictory to practical results obtained from the industry. Partly this is due to e.g. the fact that it is not possible to determine the composition and origin of an oxygen-consuming material from the results of the COD-analysis.
Thinking back, in many solutions applying a two-step oxygen stage, the reasons stated experimentally in the first stage have lead to the aim of milder delignification properties without, on one hand, exactly knowing which chemical mechanisms that is based on and, on the other hand, what will be the effect of the different origin of filtrates in this wholeness. Only experiments made in the mills have proved the solutions to be right. In practice, this has meant that the black liquor filtrate passed in form of washing loss from the cook into the two-step oxygen stage has first been oxidized around the pulp fibers in conditions moderate in view of temperature so that damages to the fibers have remained as small as possible. Not until after the above presented mild first step has it been possible to arrange the conditions in the second oxygen step so that the pulp may be delignified to a low kappanumber without adverse effect on selectivity.
One observation from the experiments is that the oxygen stage itself also produces organic compounds that have a similar catalytic effect as the cook-originating black liquor, but this chemical fraction may not actually be eliminated because it is generated into the process inside said process itself.
Solutions presented in e.g. the following patent publications represent the above mentioned two-step oxygen stages utilizing the oxidation of residual black liquor:
In the solution according to U.S. Pat. No. 5,217,575 describing a two-step oxygen stage, the required temperature difference between the first and the second step is over 20xc2x0 C. so that the first step is carried out in a lower temperature, clearly less than 90xc2x0 C. With this temperature difference, the conditions of the treatment stage are made non-advantageous for the actual oxygen stage, but based on our studies they are well suitable especially for the oxidizing of filtrates. In the modification of the two-step oxygen stage according to SE patent 505141, the oxidizing of filtrates has been solved by keeping the temperature in the first reactor, i.e. the first treatment step, below 90xc2x0 C. The solution according to FI patent publication 98224 is also aimed at the same goal.
In all these solutions, the aim has been to decrease the catalyzing effect of the cook-originating filtrate on the decomposing of the peroxide compounds by dividing the oxygen stage to two or more steps and thus to improve the quality of the pulp. On the other hand, especially in old mills, installing an oxygen stage in the mill often leads to decreased operation of the brown stock washing department, whereby the amount of cook-originating non-oxidized black liquor entering the oxygen stage is increased. In such case, quality losses caused by the oxygen stage have turned out to be unexpectedly great. In addition to that, the fluctuating running conditions of the brown stock washing department, due to e.g. various bottlenecks of the mill, and disturbances in washing conditions readily lead to increased washing losses and, accordingly, to quality losses of pulp.
That is, all the above presented solutions handling with the oxygen stage utilize the treatment of the washing liquid of the wash preceding the oxygen stage, which washing liquid thus originates from the wash after the oxygen stage, with an oxidizing chemical, or the oxidizing of black liquor filtrate in a two-step oxygen delignification together with the pulp in conditions suitable for the purpose. These solutions have their problems, too, e.g. handling the heat balance. Even without heating, the first reactor of the oxygen stage operates according to the balance at a temperature of over 90xc2x0 C. and the requirement of a lower temperature of the first reactor results in the necessity of cooling the washing liquid of the washer preceding the oxygen stage. In such a case, the pulp must be heated after the first oxygen step using high-pressure steam. Heat obtained from cooling the washing water is difficult to recover in a form preferable in view of the operational economy of the fiber line. Additionally, the investment expenses and operational expenses of heat exchangers are significant. The arrangement of temperature differences in pulp production also contributes to both the forming of precipitates and the generation of extractive problems.
As the filtrate coming from the washing of the oxygen stage is already oxidized, the treatment thereof does not significantly change the situation anymore. That is why the oxidation should according to our studies be performed before the last washing stage prior to the oxygen stage, e.g. between the last and the last but one washing stages. In this way, pulp is being displaced by filtrate, oxidized in the brown stock washing, due to which the pulp is displaced by oxidized filtrate twice (the first being filtrate led as washing liquid countercurrently from the washer following the oxygen stage and the other filtrate oxidized in the brown stock washing), which results in a significant decrease in the amount of cook-originating non-oxidized liquor. Separate oxidizing of liquor entering with the pulp is actually a modification of the oxygen stage, where separate oxidation of filtrates effects especially the properties of the filtrate travelling with the pulp and enhances the access to the aimed benefits of the two-step oxygen stage. Oxidizing the liquid solution between the washers prevents non-oxidized filtrate from entering the oxygen stage also during disturbances.
Thus, the present invention is based on the idea that filtrate essentially related to brown stock washing and the oxygen stage connected thereto is treated with an oxidizing chemical so that the aim is to shut off the black liquor flow entering with the pulp from the cook as washing loss in such a way that as much as possible of the black liquor flow travelling with the pulp in form of washing loss has been gone through an oxidizing stage prior to entering the oxygen stage.
Our studies have brought to light many new ideas concerning integrating the oxygen stage between the cooking and the washing. It has been noticed that because the pulp is hot after the cooking, typically 75-100xc2x0 C., and amply of alkali is present around the pulp, the pulp is in those conditions subjected to reaction deteriorating the fibers. No special gas dosing is need for generating these reactions, but e.g. releasing pulp from the cooking to an atmospheric state is enough to cause damages. According to our studies, pulp that had been let to stand in black liquor solution at a temperature of 90xc2x0 C. in atmospheric state under a cover was significantly deteriorated measured by viscosity, without any dosing of oxygen. Thus, alkali and cook-originating black liquor components in atmospheric state together with the oxygen of air are detrimental, so that the time between the blow of the cook and the oxygen stage should preferably be as short as possible. Accordingly, it is preferable to have directly after the cook e.g. a diffuser or DRUMDISPLACER(copyright) washer and that retention in all tanks before the oxygen stage have, especially in normal running situations, been minimized as efficiently as possible. The retention time between the blow and the oxygen stage feed might at its shortest be in the range of 1-15 minutes, by means of modem technology most probably around 10 minutes and when effected by somewhat slower alternatives most usually less than 60 minutes, i.e. in the range of 20-50 minutes. That would allow the removal of cook-originating black liquor with its solid matter as soon as possible from surrounding the fibers and replace it with oxidized filtrate originating from the oxygen stage.
Characterizing features of the present invention are described in more detail in the appended patent claims.
Utilizing the method and apparatus according to the invention, e.g. the following advantages are obtained:
The amount of black liquor catalyte entering the oxygen reactor is essentially decreased.
The oxygen stage may be carried out in conditions where the portion of non-oxidized filtrate has been significantly decreased, whereby quality losses are decreases.
Uniformity of the pulp is increased, as the amount of cook-originating black liquor is reduced.
The oxygen stage may in some cases be performed in one stage, because different conditions for oxidizing the material entering the oxygen stage as washing losses are not needed anymore.
The strength of the pulp is increased.